1. Field of the Invention
The present invention relates to a document processing system and, more particularly, to a document processing system having a French-fold printing function in which the contents of two pages of a document are printed on the left and right halves of a single sheet of paper. The term "document" used in this specification should be interpreted to generally mean various types of documents which may include at least one figure or image. The system of the invention may be composed of a plurality of separate devices, or it may be constructed as a unit.
2. Description of the Prior Art
Document processing systems such as word processors are known. These processing systems have various printing functions. One of these known printing functions is the so-called French-fold printing function in which the contents of two pages of a document formed by the processing system are printed on the left and right halves of a single sheet of paper. It is assumed here that the thus-formed document is stored in a document memory as two pages, i.e., a first page 20 and a second page 21, as shown in FIG. 2. FIG. 3 shows the result of French-fold printing on a single paper sheet 30. It will be seen that the contents of the first page 20 and the second page 21 are printed on the left and right halves of the paper 30, respectively.
Most of the document processing systems of this kind employ serial printers such as wire dot-matrix printers or thermal printers. Such a serial printer is usually designed so that it prints in a line-by-line fashion. In the French-fold printing mode, therefore, the printer prints the corresponding lines of both pages, e.g., the ith line of the left page and the ith line of the right page, in a single stroke. For instance, in the illustrated case, the third line 40 on the left page and the third line 41 on the right page are formed into a dot pattern 42 as shown in FIG. 4, and the printer operates in response to this pattern to print those lines in one stroke.
Some of the document processing systems of the type described above have a function which enables the user to select the line feed or change pitch for each line. When printing is conducted by such a system in the French-fold printing mode, it is impossible to print the corresponding lines of both pages with independently changing line pitches. In other words, when a changed line pitch is selected for one of the two pages, the printing of the other page is inevitably conducted with that changed line pitch. In the French-fold printing mode, therefore, the user cannot enjoy the advantage of the function of selecting a changed line pitch for each line.
The editing of a document often requires that the printhead is located in relation to a print position on the preceding line, but only on the left or right page. In other cases, both pages are treated as a single page and the printhead is located in relation to a print position on the preceding line which is composed of corresponding lines of two successive pages. Unfortunately, known document processing systems cannot meet these requirements.